The invention relates to a method and an apparatus for guiding a moving web of material. More particularly, it relates to such a method and apparatus for guiding a moving web of material having print marks at least over stretches of the web, in particular, a paper web or web of foil material, in a web-guiding device in which provision is made for an adjustable correcting device for correcting the web alignment if the center of the web runs off course relative to a nominal running line. The correcting device is connected via a controlling system to an edge-sensing system, whereby over print mark stretches of the web, the relative position of the print marks with respect to an imaginary guide line can be checked and print marks signals superimposing the correcting signals of the edge-sensing system can be entered in the controlling system for additional correction, if required.
Normally, webs of paper or foil material having print marks over at least stretches of the web are controlled with respect to the center of the material web by means of an edge-sensing system (see, e.g., U.S. Pat. No. 4,485,982). However, unpredictable influences such as temperature and moisture or a general shrinking of the web of material lead to a reduction in the spacing of the print marks from the center of the web, so that with the method controlling the center by means of the edge-sensing system, the print marks no longer run in an imaginary guide line. As the print marks serve as reference marks for imprints to be applied to the web, this means that blurred print patterns are produced, which leads to web rejects. For this reason, with the known method, an operator monitors the quality of the printed pattern and interferes with the control process if the printed pattern starts to become blurred. For example, the operator enters in the control units correction pulses which are directed against the side on which the print marks run off course relative to the imaginary guide line, and the web of material is adjusted sideways via the correction system until the printed pattern is sharp again. This, however, is a highly troublesome method that requires constant monitoring by a skilled operator and, furthermore, represents an improvisation not always leading to acceptable results.